A radio frequency (RF) transmission line is a structure that is designed to efficiently transmit high frequency, radio frequency (RF) signals. An RF transmission line typically comprises two conductors, such as a pair of metal wires, which are separated by an insulating material with dielectric properties, such as a polymer or air. One type of an RF transmission line which is well known in the art is a coaxial electric device.
Coaxial electric devices, such as coaxial cables, coaxial connectors and coaxial switches, are well known in the art and are widely used to transmit electromagnetic signals over 10 MHz with minimum loss and limited distortion. As a result, coaxial electric devices are commonly used to transmit and receive signals used in telecommunications, broadcast, military, security and civilian transceiver applications, as well as numerous additional uses.
A coaxial electric device typically comprises an inner signal conductor which serves to transmit the desired frequency communication signal between a source and a load. The inner signal conductor is separated from an outer conductor by an insulating material, or dielectric material, the outer conductor serving as the return path, or ground, for the communication signal. Such an electric device is typically referred to as coaxial because the inner and outer conductors share a common longitudinal axis. It should be noted that the relationship of the geometry of the conductors and the properties of the dielectric materials disposed between the conductors substantially define the characteristic impedance of the coaxial device.
It has been found that, on occasion, potentially harmful voltages are transmitted through RF transmission lines. In particular, radios operating in either the lower end of the ultra high frequency (UHF) band or lower frequency bands (i.e., below 500 MHz) often utilize longer antenna lengths to enhance performance when compared to antennae used in higher frequency applications. Furthermore, since the mounting height of a radio antenna serves to increase its range, radio antennae are commonly mounted from an elevated position (e.g., a tower or mast). As a result, it has been found that radio antennae are highly susceptible to lightning strikes, the high electrical energy of a lightning strike increasing the likelihood of significant damage to any sensitive components and circuits connected to the transmission line.
A coaxial protective device, or coaxial protector, is commonly incorporated into an RF transmission line in order to suppress or otherwise deflect undesirable electromagnetic impulses away from a load connected thereto. Coaxial protectors are typically designed to include one or more protective components in order to treat undesirable electromagnetic signals.
For instance, one type of coaxial protective device which is well-known in the art conductively couples at least one voltage suppression component, such as a gas discharge tube, between the inner signal conductor and the grounded outer conductor. Accordingly, excessive voltage (e.g., as a result of a lightning strike) transmitted along the inner conductor is diverted from the inner conductor and treated by the voltage suppression device, thereby protecting sensitive equipment connected to the transmission line.
Although well-known and widely used in the art, coaxial protective devices of the type described above, which dispose at least one voltage suppression component between the inner and outer conductors, often introduce disturbance to the transmission line. In particular, gas discharge tubes used in coaxial protective devices are traditionally large in size, with a relatively wide diameter in transverse cross-section (approximately 8 mm) and a comparatively shorter length (approximately 6 mm). Because the inner conductor is formed using a conductive pin of limited diameter (approximately 3 mm) to minimize the overall size of the protector, the pin is often effectively widened to a considerable degree in diameter (approximately 8 mm) in the region of contact with the gas discharge tubes as a result of the end electrode dimensions of the gas discharge tubes or to ensure that an adequate conductive path is established. Due to both the considerable widening of inner conductor pin (often by a factor of 2 or more) as well as the inherent capacitance of the one or more gas discharge tubes, disturbance is imparted onto the transmission line, which is highly undesirable. Although certain attempts have been made in the art to reduce transmission line disturbance caused from the inclusion of voltage suppression devices, most solutions result in either a substantial increase in the overall size of the protective device and/or suffer a reduction in the upper operational frequency.
Another type of coaxial protective device which is well-known in the art incorporates a signal treatment component (e.g., a capacitor, resistor, inductor, fuse or semiconductor) directly into the inner conductor. For instance, the inner conductor may include multiple conductive elements (including the signal treatment component), which are joined end-to-end, typically using a compressive force, to form a unitary, linear, conductive member.
Although well-known and widely used in the art, coaxial protective devices of the type described above, which incorporate a signal treatment component directly into the inner conductor, often require use of a resilient member, such as a coil spring, to compensate for variances in inner conductor geometry that result from, inter alia, tolerances in manufacturing as well as certain environmental conditions (e.g., changes in temperature). However, the use of a coil spring in the assembly of the inner conductor has been found to be undesirable due to the self-inductance of the coil. In addition, a coil spring typically establishes an area of contact with adjacent components that is significantly narrower than the remaining center pin diameter. This dimensional reduction in the current path along the inner conductor creates a variance in impedance along the RF transmission line, which is highly undesirable.
Another type of coaxial protective device which is well-known in the art includes a shunt conductor that connects the center conductor to ground. A quarter-wave stub or an inductor is typically utilized as the shunt conductor in this type of protective device. Although widely used in the art, this type of coaxial protector has been found to suffer from certain limitations. Specifically, the protector has been found to provide either a limited operational frequency of over 400 MHz when a quarter-wave stub is utilized or a narrow banded performance when an inductor is utilized. In addition, due to the relatively high impedance and physically longer length of the shunt conductor, the protective device affords poor protection in the lower frequency range.